Several substantial vaccine efforts against HIV-1 have so far failed primarily because to date we have failed to identify the correlates of protectiv immunity and the optimal vaccine formulation that can induce such protective immune responses in vivo. The knowledge that only select single or limited virus species are transmitted via the mucosal route has advanced the concept of Transmitter/Founder viruses (T/F). It is reasoned that such viruses that are preferentially transmitted should be the target of HIV vaccine efforts. The clade 'C' viruses that constitute the major clade transmitted sexually worldwide therefore have become the focus of studies for vaccine formulations. However, there is a lack of both suitable clade 'C' viruses and an optimal nonhuman primate model that faithfully mimics such natural transmission so that it can be utilized for the testing of candidate vaccines or microbicides. The present proposal is directed at providing the initial foundation for these objectives. Our lab has available unique pairs of infectious molecular clones (IMC) of replication competent T/F viruses from heterosexual transmission studies being conducted in Rwanda-Zambia. Our lab has prepared and tested a number of SHIVs in the past and is thus highly experienced to exploit these unique sets of T/F cloned viruses and prepare in vitro and in vivo replication competent clade C T/F env-SHIVs which will share all the properties required for the testing of candidate vaccines. We plan to carry out systematic studies that include 1) the derivation and the detailed in vitro characterization of such recombinant SHIVs using a battery of tests 2) To utilize novel in vivo cell lineage depletion techniques that our lab has optimized i rhesus macaques and attempt to adapt the SHIVs for efficient replication in such models leading to the isolation of swarms of SHIVs stocks and 3) to test such in vitro and in vivo replication competent swarms of SHIV's for mucosal transmission. We submit that we are uniquely poised in terms of reagents, talent, experience and knowledge to achieve the objectives outlined in this proposal.